Q: I’m currently dealing with insulating a cathedral ceiling, which has become a major issue. I’ve been researching solutions but feel stuck in figuring out what’s the best way to go about dealing with the problem.
I have a shed roof with 2:12 pitch, built with 2×8 roof joists. It’s a mobile home that’s 14×76, so the roof is just one continuous slope. I live in northern Oregon with lots of rain in my area and occasional freezes.
The issues began when I discovered that my newly installed asphalt shingles never got sealed by sun exposure, creating a massive roof leak. I ended up gluing them with Henry’s Wet Patch adhesive, which seems to have solved that problem.
The second issue is that I’m one of those people who was after the highest R-value in the ceiling. So I used R-30 faced fiberglass insulation which didn’t leave any vent space beneath the roof sheathing. I pulled the insulation out and used plastic vent channels beneath the sheathing, then stuffed R-30 back in there, compressing it a little.
A few days later I checked, and saw that there was lots of moisture accumulation on the bottom of those plastic spacers. I’m talking about visible large droplets along the entire bottom surface of the spacers!
I want to mention that I do have vent holes in both bottom and top eaves. After reading your comments on a previous cathedral ceiling question, I’m wondering if I should thin out the R-30 batts to where it is closer to R-15, take those plastic spacers out, and then screw foil-faced foam board beneath the roof joists.
Or should I just take the fiberglass out and use only foam boards. Another thing I’m considering is whether the moisture that I’m seeing is from roof sheathing drying after that roof leak…
I’m about to re-do the ceiling for the third time and really want It to be finally done right. Please advise if you can. Thank you – I appreciate your time! — Ilya
A: Let’s start at the top. Most asphalt roofing manufacturers recommend a minimum slope of 4:12. For roofs between 2:12 and 4:12, they typically recommend double-coverage underlayment (that is, lapped 19 in. for a 36 in. roll) or peel-and-stick membrane, which would be my preference, especially in a wet climate like yours. So the water you are seeing on your plastic spacers could be from roof leakage.
Most likely, however, the water droplets are condensation of moisture from inside the house. Warm interior air is always moving upward in cold weather due to the stack effect. The plastic vent baffles are the first cold surface reached by moist interior air that leaks into the roof structure in cold weather.
So the plastic vent baffles become the “plane of condensation”. Whenever a cold surface is below the dew point of the surrounding air, condensation will occur. Think cold beer can on a hot, humid day.
In a small house like yours, the indoor humidity is often high when the house is closed up in cold weather. Spot ventilation can help, especially in the kitchen and bathrooms. If you have condensation running down the inside face of double-pane windows, that’s an indication that indoor humidity is too high. If spot ventilation is not enough, whole-house ventilation may be needed. A timer on a bath fan is the simplest, cheapest approach.
The other key to keeping moist air out of your roof structure is an airtight air barrier at the ceiling level, below the insulated rafter cavities. This can be achieved with careful sealing of the drywall, or with foam-board insulation that is taped at seams and sealed around the perimeter with canned foam.
Roof ventilation also helps, but it should not be the main defense against condensation under the sheathing. Its main purpose is to reduce ice dams in winter and reduce overheating of the roof in summer. It may increase the lifespan of roofing shingles. You should definitely keep the roof ventilation unless you are using a “hot roof” design with closed-cell spray foam – a good, but expensive approach.
With moderate indoor humidity and a tight ceiling air barrier, you can use whatever type of insulation you want. If you have the headroom to place 1-2 in. foam boards below the joists, that’s a good way to go. If you use 1 in. foil-faced foam (R-6-7) plus a 6-in. R-19 batt, you’re getting close to the R-30 minimum for roof insulation required by many codes. It also greatly reduces thermal bridging though the joists. Any type of foam board will work.
The R-19 batts leave 2 in. for the recommended vent space under the roof sheathing. High-density 6-in. batts provide R-21. If you can add an air space below the foil facing by furring down the drywall, you get an extra R-3 approximately.
There’s nothing magical about the R-30 number, which is built into the codes based on the low cost of attic insulation, not the higher cost of cathedral ceiling insulation. It has nothing to with the fact that “heat rises” as many people believe. But it is the law in many locales. — Steve Bliss, BuildingAdvisor.com
See also Sealing an Unvented Cathedral Ceiling Insulating Cathedral Ceiling With Foamboard
LR says
Best Insulation For 2×6 Rafters?
We bought a 450 sq ft cottage in Buffalo, NY, and are remodeling it. The walls have insulation, but not the roof. It is low-pitched with no ridge vent, and only 2×6 rafter spaces for insulation. We bought R-30 insulation and baffles, but the R-30 is too thick to fit. The plan to have a cathedral ceiling finished with drywall.
We think the structure was originally built as a garage and converted into a cottage. We don’t plan to live there year-round, but it will be using it as a getaway, mostly in the summer. It will have small gas wall heater for cold-weather use.
What’s the best type of insulation to use?
buildingadvisor says
If this was year-round living space and you wanted (or needed) to achieve the typical code minimum of R-30, then your best bet would be an unvented “hot roof” using closed-cell urethane spray foam. This provides about R-6 per inch and provides an effective air seal to keep household moisture out of the roof cavities. It is a one-step solution, but also the most expensive. It is probably overkill for a vacation cottage used mostly in fair weather.
The least expensive option is batt insulation, but that requires vent channels of 1-2 inches below the sheathing – 2 in. preferred. That leaves you with R-11 or R-13 (high density) batts. Also, you will need to retrofit a ridge vent, not a difficult job in warm weather.
The simplest way to beef this up, if you have the headroom, is to add a layer of foam board under the rafters. Adding foam board adds R-value and reduces the thermal bridging through the rafters. By taping the seams and used canned foam around the perimeter, the foam can also serve as your air barrier.
An airtight air barrier is needed to keep moist household air out of the roof structure. One inch of foam would add from R-4 to R-7 depending on the type of foam. With two inches of foil-face isocyanurate foam, you be just about at R-30. If you add a layer of ¾ strapping before installing the drywall, you will get an additional R-3 for the air space facing the foil.
One more option to consider, if you plan to do the work yourself, is called “cut and cobble”. That means cutting strips of foam board to fit tightly between the rafters. You could do this as either a vented or hot roof. You will need to very carefully tape and seal the bottom layer of foam board to create an airtight air seal – critical if this is a hot roof. For the best performance, you would want each layer to fit tightly, foaming any cracks around the perimeter and taping seams.
Cut and cobble is very labor intensive, but doable, once you work out a good method for accurately cutting the foam. I’ve tried table saws, saber saws, serrated knives, hand saws, long-blade razor knives, etc., with mixed success. None works as well as a professional foam cutting tool.